As manufacturing apparatuses for use in the fields of manufacturing electronic devices such as semiconductors and flat panel displays, and so on, there are, for example, vacuum thin-film forming apparatuses for use in chemical vapor deposition (CVD), physical vapor deposition (PVD), vacuum evaporation, sputtering, microwave-excited plasma CVD, and so on, dry etching apparatuses for use in plasma etching, reactive ion etching (RIE), recently-developed microwave-excited plasma etching, and so on (hereinafter collectively referred to as vacuum apparatuses), cleaning apparatuses, sintering apparatuses, heating apparatuses, and so on. In recent years, as structural materials of these apparatuses, particularly having surfaces brought into contact with corrosive fluids, radicals, or irradiated ions, lightweight and strong metals composed mainly of aluminum have been widely used instead of stainless materials. In order to realize future efficient multi-kind small-quantity production, these apparatuses are required to shift to a three-dimensional cluster tool capable of carrying out a plurality of processes alone, to carry out a plurality of processes by switching the kind of gas in a single process chamber, or the like. Among practical metals, aluminum belongs to a particularly base group and, therefore, aluminum or a metal composed mainly of aluminum requires a protective film formed by an appropriate surface treatment.
As a surface protective film when a metal composed mainly of aluminum is used as a structural material, there is conventionally known an anodized film (alumite) obtained by anodic oxidation in an electrolyte solution. If use is made as the electrolyte solution of an acidic electrolyte solution (normally pH 2 or less), it is possible to form a smooth and uniform alumite coating film having a porous structure.
Further, the alumite coating film is corrosion-resistant and the acidic electrolyte solution is stable and easy to manage, and therefore, the alumite coating film is generally and widely used. However, the alumite coating film having the porous structure is weak against heat as a surface treatment of a structural member and thus causes cracks due to a difference in thermal expansion coefficient between the aluminum base member and the alumite coating film (Patent Document 1—Japanese Unexamined Patent Application Publication (JP-A) No. H10-130884), thereby causing generation of particles, occurrence of corrosion due to exposure of the aluminum base member, and so on.
Further, a large amount of water and so on is accumulated/adsorbed in holes of the porous structure (Patent Document 2—JP-B-H5-053870), which is released in large quantities as outgas components. This causes many problems such as a significant reduction in performance of a vacuum apparatus, operation failure of devices, occurrence of corrosion of the alumite coating film and the aluminum base member due to coexistence with various gases including a halogen gas and chemicals, and so on. Among halogen gases, particularly a chlorine gas is used as an etching gas in the processing, such as reactive ion etching (RIE), of a metal material and is also used in a cleaning process of a thin film forming apparatus or a dry etching apparatus and, therefore, it is important to achieve a metal surface treatment of an apparatus member that can ensure strong corrosion resistance against the chlorine gas.
In view of this, there are various proposals for alumite coating films with a low increase rate of cracks caused by a high-temperature heat load and their forming methods. For example, there is proposed a method of forming an alumite coating film with a controlled aluminum alloy composition (Patent Document 3—JP-A-H11-181595). However, this alumite coating film also has a porous structure on its surface like the conventional one and various problems due to water remaining in holes of the porous structure remain outstanding.
Various methods are proposed for improving the problems caused by this porous structure. For example, there are proposed a sealing treatment in which an alumite coating film with a porous structure anodized in an acidic electrolyte is immersed in boiling water or treated in pressurized steam, thereby forming aluminum hydroxide (boehmite layer) on its surface to seal holes thereof (Patent Document 4—JP-A-H5-114582), a sealing treatment in a solution containing mainly a hydrate or hydrated oxide of a metal (Patent Document 5—JP-A-2004-060044), and so on. However, water still remains in the holes of the porous structure even after the sealing treatment and the boehmite layer of aluminum hydroxide itself is also a hydrate and thus serves as a water supply source depending on the conditions such as a pressure and a temperature and, therefore, a radical solution has not yet been reached. There is also proposed a method of performing barrier-structure anodic oxidation after forming a porous-structure alumite coating film (Patent Document 6—JP-A-2005-105300). However, since anodic oxidation in two steps is required to be performed, there is a problem that the manufacturing cost increases.
Besides, as a surface treatment when a metal composed mainly of aluminum is used as a structural member, use is made of a thermal spraying method that melts and sprays a powder material of a metal, an alloy, a ceramic, or a combination of the ceramic and the metal or the alloy (Patent Document 7-JP-A-H9-069514). However, in the surface treatment by the thermal spraying method, there remains a problem in that since it is difficult to suppress formation of pores where the film surface and the base member communicate with each other through holes, when a corrosive gas such as a halogen gas is used in an apparatus, the base member is corroded at portions where the metal composed mainly of aluminum is brought into contact with the corrosive gas through the pores.
In order to solve the above problems, the inventors have found that when a metal composed mainly of aluminum is anodized using an anodizing solution with a neutral or nearly neutral pH, an aluminum oxide passivation film being a nonporous amorphous film is obtained such that the water release amount from the aluminum metal oxide film is 1E18 molecules/cm2 or less, and the film is free from occurrence of cracks due to annealing and excellent in resistance against exposure to a chlorine gas (Patent Documents 8 and 9). Further, the inventors have carried out anodic oxidation with respect to high-purity aluminum in which the total content of specific elements (Fe, Cu, Mn, Zn, and Cr) of an aluminum alloy composed mainly of aluminum is suppressed to 1 wt % or less, Mg is added in an amount of 4.5 wt % or less, and Zr is added in an amount of 0.1 wt % and with respect to 99.99% (4N) aluminum in which the content of specific elements is 0.004 wt % and no Mg or Zr is contained, and have found that it is possible to achieve anodization up to a predetermined voltage in a shorter time and the residual current density is small in oxidation at a constant voltage, and thus, it is possible to form excellent barrier-type aluminum oxide films at high throughput. Further, the inventors have found that these high-purity aluminum oxide films are excellent in resistance to a chlorine gas (Patent Document 10).    Patent Document 1: JP-A-H10-130884    Patent Document 2: JP-B-H5-053870    Patent Document 3: JP-A-H11-181595    Patent Document 4: JP-A-H5-114582    Patent Document 5: JP-A-2004-060044    Patent Document 6: JP-A-2005-105300    Patent Document 7: JP-A-H9-069514    Patent Document 8: Japanese Patent Application No. 2005-178562    Patent Document 9: Japanese Patent Application No. 2006-64923    Patent Document 10: PCT International Application No. PCT/JP2006/309327